The present invention relates to an ultrasonic rotary horn. More particularly, the present invention relates to an ultrasonic rotary horn having unique characteristics when operated at a frequency in the range of from about 18 to about 60 kHz.
The use of ultrasonic energy to bond and/or cut thermoplastic materials on a continuous basis is well established, with one of the earliest references being British Patent No. 1,018,971 to Bull which issued in 1966. Applications include the continuous seaming or point bonding of roll goods (Canadian Patent No. 1,092,052 to USS Engineers and Consultants, Inc.), the ultrasonic bonding of materials to form a pattern in a multilayer web which subsequently is cut out of the web (U.S. Pat. No. 3,562,041 to Robertson), the sealing of the ends of individual absorbent products (U.S. Pat. No. 3,677,861 to Knauf), the patterned lamination of webs of nonwoven fabric, fiberfill, and woven shell fabric to produce mattress pads and bedspreads (U.S. Pat. No. 3,733,238 to Long et al.), and the simultaneous bonding and cutting of two webs to form gloves (U.S. Pat. No. 3,939,033 to Grgach et al.).
Many applications of ultrasonic energy for the bonding and/or cutting of thermoplastic materials involve ultrasonic horns or tools which are stationary (i.e., nonrotating), in which the direction of application of the horn working surface is coincident with the direction of the applied mechanical vibrations. Such horns most commonly are stepped cylinders or stepped blades. Thus, the working surface of the horn is moved axially against a rigid anvil of suitable design, with the materials to be bonded or cut being positioned between the horn and the anvil.
In an interesting variation of the rigid horn configuration, one reference is known which discloses a horn which does not come in contact with the materials to be bonded. See U.S. Pat. No. 4,668,316 to Sager.
Another configuration, which is more conducive to continuous high-speed bonding operations, is that of a stationary horn and a rotating anvil; see, by way of illustration, U.S. Pat. Nos. 3,562,041, 3,733,238, and 3,939,033, infra, U.S. Pat. Nos. 3,844,869 to Rust, Jr. (apparatus for ultrasonic welding of sheet materials), 3,993,532 to McDonald et al. (ultrasonic sealing pattern roll, i.e., patterned rotating anvil), and 4,659,614 to Vitale (ultrasonically bonded nonwoven fabric through the use of a patterned rotating anvil), and German Published Patent Application No. 2,259,203 to J. H. Benecke GmbH (improvement of physical properties of nonwoven materials by ultrasonic bonding). In this configuration, the materials to be bonded are passed continuously between the horn and the rotating anvil. The linear velocity of the materials typically is equal to the tangential velocity of the working surface of the rotating anvil. See, also, U.S. Pat. No. 3,575,752 to Carpenter in which a rigid anvil is employed immediately before a drive drum.
Although the use of a rotating anvil was a significant improvement in continuous bonding processes involving the application of ultrasonic energy, such use has some inherent limitations which adversely affect line speed and bonding quality. It is, of course, necessary to continuously pass the materials to be bonded between the narrow gap formed by the rotating anvil and the rigid, stationary horn. This often leads to a damming effect at the leading edge of the horn, as well as to possible compression variations due to nonuniformities in material thickness. Hence, there is created a stick-slip condition which strongly influences the efficiency of acoustic energy transfer. This greatly affects the resulting bond quality which in turn limits line speeds. This phenomenon also limits the compressible bulk or thickness of the materials to be bonded.
One approach to diminish the extent of these limitations has been the development of the arcuate profiled horn which gives a progressive convergent-divergent gap. See, for example, U.S. Pat. No. 4,404,052 to Persson et al. Another approach has been to orient a modified stepped-blade horn parallel with and between the sheets to be bonded as shown in U.S. Pat. No. Re. 33,063 to Obeda. It is apparent, however, that as long as a stationary horn is used, the problems associated with movement of the materials to be bonded past the horn cannot be eliminated entirely in view of the fact that intimate contact is necessary for efficient acoustic energy transfer.
The approach which appears to have received the most attention is the use of a rotating (rotary) horn in conjunction with a rotating anvil. Such a configuration is best described as two or more cylindrical surfaces which are in close proximity to each other and rotating in opposite directions with equal tangential velocities. The materials to be bonded pass between the cylindrical surfaces at a linear velocity which is equal to the tangential velocities of such surfaces. Thus, the opportunity for damming and stick-slip is virtually eliminated.
The concept of an ultrasonic rotary horn, typically used in conjunction with a rotating anvil for the bonding of a variety of materials, is, of course, well known. See, by way of illustration, U.S. Pat. Nos. 3,017,792 to Elmore et al. (vibratory device), 3,217,957 to Jarvie et al. (welding apparatus), 3,224,915 to Balamuth et al. (method of joining thermoplastic sheet material by ultrasonic vibrations), 3,272,682 to Balamuth et al. (apparatus for joining thermoplastic sheet material), 3,455,015 to Daniels et al. (ultrasonic welding method and apparatus), 3,623,926 to Sager (method and apparatus for the assembly of thermoplastic members), 3,955,740 to Shoh (vibratory seam welding apparatus), 4,252,586 to Scott (method and apparatus for the manufacture of slide fastener stringer with folded and bonded continuous molded coupling elements), 4,333,791 to Onishi (ultrasonic seam welding apparatus), and 4,473,432 Leader et al. (dot heat stapling); Russian Patent Nos. 178,656 (ultrasonic seam welder), 272,020 (ultrasonic seam welding unit), 277,524 (ultrasonic metal welder), 524,699 (ultrasonic seam welder for plastics), 670,406 (apparatus for ultrasonic seam welding), and 785,051 (equipment for seam welding polymeric materials); Japanese Published Patent Application Nos. 51-69578 (oscillator system for continuous ultrasonic welding of plastic) and 58-42049 (continuous ultrasonic jointing device for photographic papers); French Patent No. 1,459,687 (ultrasonic welding of an aluminum foil onto a glass sheet -see also Japanese Patent No. 42-22222); German Published Patent Application No. 3,147,255 to Licentia Patent-Verwaltungs-GmbH (apparatus for welding solar cell contacts and connectors); and Australian Patent No. 260,888 to Knudsen et al. (ultrasonic welding rollers for use in making a metal container).
Nonbonding applications involving rotary horns also are known, some examples of which are included here for completeness: U.S. Pat. Nos. 3,096,672 to Jones (vibrating roll and method), 3,292,838 to Farley (rotating sonic welder), 3,550,419 to Fox et al. (methods and apparatus for metal rolling), 3,620,061 to Cunningham et al. (design of ultrasonic transducers for use with rolling mill rolls), 3,678,720 to Dickey et al. (roller leveler and method of leveling), and 3,908,808 to Busker (ultrasonic calendaring of paper webs).
One commercially available bonding system employing an ultrasonic rotary horn is known. This system, manufactured by Mecasonic, a French firm (available from Mecasonic-KLN, Inc., Fullerton, Calif.), exploits the so-called Poisson coupling effect. Maximum radial displacement is achieved at a region of minimal longitudinal displacement which is one-quarter wavelength from the free end of a one-wavelength circular shaft that is ultrasonically excited longitudinally, i.e., along its axis, by a piezoelectric crystal assembly. An ultrasonic rotary horn which appears to be essentially the same as the Mecasonic horn is described in Japanese Published Patent Application No. 51-69578, mentioned earlier. See also European Patent Application No. 88402437.3, Publication No. 0 313 425 Al to Societe Mecasonic, which describes the use of the Mecasonic horn in conjunction with a rotating anvil or support to smooth freshly stitched seams in flexible materials.
The larger diameter or disk portion of the horn is situated at a longitudinal node where the radial displacement of the disk portion is maximum, i.e., at the radial antinode. The entire length of the horn is equal to one wavelength and the shorter distance from the center of the disk portion to the free or nondriven end of the horn is one-quarter wavelength. Upon subjecting the horn to ultrasonic excitation at the driven end, when the source of excitation moves toward the driven end of the horn, the entire horn moves longitudinally away from the source of excitation while the radial surface moves inwardly. When the source of excitation moves away from the driven end, the entire horn moves toward it while the radial surface moves outwardly. While the radial motion is at a maximum at the center of the disk portion, it diminishes rapidly in either direction across the surface of the disk portion with increasing distance from the longitudinal node (radial antinode). In cases where bond strength is at least in part a function of amplitude, this variation in amplitude must result in a variation in bond strength. Hence, the useful width of the disk portion may be reduced to something less than the actual width of 30 mm if a uniform bond strength is desired across the width of the bond zone. Such nonuniformity clearly is undesirable unless the minimum bond strength achieved will withstand the stresses placed upon the bond zone.
Thus, there is a need for an improved ultrasonic rotary horn which can be operated at an excitation frequency of from about 18 to about 60 kHz and which will provide a relatively wide bonding or other processing span with relatively constant amplitude characteristics across the width of the radial surface, and improved performance, as well as other beneficial characteristics.